1. Field of the Invention
The present invention relates to an exhaust pipe for mounting an oxygen sensor and more specifically relates to an exhaust pipe which includes a dual portion where an oxygen sensor is installed and which is connected to a dual exhaust manifold of an internal combustion engine.
2. Description of the Prior Art
An exhaust pipe connected to a dual exhaust manifold of an internal combustion engine includes a dual portion having two paths therein at an upstream portion thereof. When installing an oxygen sensor on the exhaust system including the dual exhaust manifold and the dual exhaust pipe, the oxygen sensor must be disposed at a communicating portion of the two paths or at a single path portion of the exhaust pipe which is connected to a downstream side end of the dual portion of the exhaust pipe, in order that the exhaust gas flowing from every cylinder of the engine can contact the oxygen sensor.
FIGS. 7-10 show one example of prior art arrangement of the oxygen sensor which is disclosed in Japanese Utility Model Publications Nos. SHO 61-19754 or SHO 60-3225. As shown in FIGS. 7-10, in the prior art the communicating portion 23 of the two paths are formed in the furthest upstream portion of the two paths, that is, in the dual exhaust manifold 21 for the purpose of preventing the temperature of the exhaust gas contacting the oxygen sensor from being excessively decreased, and the oxygen sensor 22 is installed at the communicating portion 23.
However, this prior art arrangement of the oxygen sensor 22 has the following drawbacks. Since the exhaust gas flowing from every cylinder has not yet been sufficiently mixed and tends to flow along the outside wall portions of the bent ports as shown by the flow lines 24 in FIG. 7 and such flow 24 can not contact the oxygen sensor 22, accurate detection of the oxygen contained in the exhaust gas flowing from every cylinder by the oxygen sensor 22 has been impossible. Further, since the exhaust gas flowing in the exhaust manifold 21 still retains too high temperatures, the oxygen sensor 22 easily suffers thermal damage as well as decreased durability.
Although contact of the oxygen sensor by the exhaust gas may be improved by providing an agitator for mixing the exhaust gas upstream of the communicating portion, such an agitator inevitably increases the back pressure of the engine as well as decreases the dual effect of the dual structure.